The diploma thesis deals with the activity of 137Cs in the organs of Vaccinium myrtillus in the mountain areas of Slovakia. The 137Cs radioisotope got into the environment as a result of nuclear weapon tests between 1945 and 1963 and accidents of nuclear power plants. The greatest impact on the territory of present-day Slovakia has the accident at the nuclear reactor in Chernobyl on 26 April 1986, which significantly contaminated the area. Radiocaesium has a half-life of 30.17 years, and is still detectable in components of the biosphere, especially in mountainous areas. Due to the ability to efficiently accumulate radiocaesium, bilberry (Vaccinium myrtillus L.) has become a reference material for the accurate determination of radiation contamination. The aim of this work was to analyse the issue of environmental contamination with emphasis on the content of 137Cs in mountain areas and the behaviour of 137Cs in the soil - plant - fruit system. Another intention was to take, modify and analyse samples according to the prescribed methodology, then process the results and discuss in the context of results published by other authors. Samples of fruits, stems and leaves of the bilberry plant and samples of the soil were taken in several mountain areas of Slovakia. Mass activity of samples were determined by gamma spectrometric analysis. The average mass activity of the samples was 59.5 ± 3.6 Bq.kg-1 in the fruits, 52.0 ± 3.3 Bq.kg-1 in the stems, and the highest activity was found to be in the leaves of the bilberry 69.9 ± 5.5 Bq.kg-1. In average, soil samples showed activity of 74.6 ± 4.3 Bq.kg-1. Area activity of the soil was determined for the locality of Flochová for the years 2017 and 2019. The sample from 2017 showed almost twice the area activity of 137Cs, but the aggregated transfer factor of radiocaesium from soil to fruits was almost tripled in the sample from 2019. The average of the aggregated transfer factors of individual parts of the bilberry from Flochová is in the range of European averages. The annual effective dose by consumption of 0.61 kg of fruit and tea from 3 g of contaminated leaves was also determined in all sampled localities. Consumption of a given amount of fruit would contribute to the effective annual dose on average by 3.28 × 10-8 mSv.year-1 and consumption of the mentioned tea on a daily basis would contribute to the permissible dose of 1 mSv by 6.74 × 10-6 mSv.year-1.